This invention relates to plant micronutrient solutions, especially to such solutions containing manganese.
In addition to treating soils for agricultural use with fertilizers containing nitrogen, phosphorus and potassium, it has become common practice to apply certain micronutrient elements essential for proper plant nutrition, including iron, zinc, manganese and copper, to correct or prevent deficiencies of such elements. Such elements are most conveniently applied in aqueous solutions either to the soil in which the plants are grown or to the plant itself as a foliar spray. However, it has been found that aqueous solutions of manganese micronutrients present certain problems in storage and application. Firstly, it is necessary that the manganese be in solution and applied to the plant as a soluble form of manganese. It is therefore important to avoid precipitation of the manganese during storage or on application. Further, it is most convenient and economical for the user to apply several fertilizer components, pesticides or other agricultural chemicals in a single operation. Accordingly, it is desirable to provide the manganese in a solution form that is compatible with other fertilizers, particularly the widely used nitrogen-phosphorus-potassium liquid fertilizers and with other agricultural chemicals. Aqueous solutions of manganese salts such as manganous sulfate form insoluble complex salts when combined with liquid nitrogen-phosphorus-potassium fertilizers and accordingly separate application of the fertilizer and manganese solution has been necessary. Efforts to provide divalent manganese solutions in a form compatible with liquid fertilizers have typically involved complexing the manganous ion with a chelating agent such as EDTA or a polyhydroxy acid. U.S. Pat. No. 3,869,272 discloses an ammoniacal aqueous solution having a pH in the range 9 to 12 and containing 3 to 8% soluble divalent manganese, at least 1 mole of citric acid per mole of manganous ion, together with about 0.1 to 5 wt. % of sodium bisulfite as an oxidation inhibitor. Other suitable antioxidants for use in such a solution are sulfites, dithionites, thiosulfates and hydroxylamines.